1. Field of the Invention
The present invention relates to a method for continuous casting of cast products and an apparatus therefor, and particularly relates to a method for horizontal continuous casting of metal strip cast products having a completely unidirectional solidified structure which is wholly elongated in the direction of casting by use of a substantially horizontally disposed gutter like hot mold and an apparatus therefor.
2. Description of the Related Art
A conventional method for horizontal continuous casting of cast products is a method in which a through hollow cold mold is horizontally disposed, a molten metal is supplied from one end of the mold, the supplied molten metal is solidified in the mold, and a cast product is continuously drawn out from the other end of the mold. This method has been widely used for performing casting to obtain cast products of an iron alloy, an aluminum alloy, a copper alloy, or the like.
However, this method has such disadvantages that the molten metal supplied into the mold forms a solidified shell along the wall surface of the mold, and a not-yet solidified molten metal in the inside enclosed by the soldified shell is completely solidified through secondary cooling at the outside of the mold, so that impurities are concentrated to thereby generate defects such as component segregation and/or bubbles in the finally solidified portion at the center of the cast product.
Further, in such a conventional method, intermittent drawing in which a stably solidified shell of a cast product coming out from the mold is grown and then the cast produce is drawn out has been performed in order to prevent occurrence of surface cracks due to friction between the mold and the surface of the cast product in drawing the cast product and breakout of the molten metal. However, oscillation marks formed in the surface of the cast product through intermittent drawing may cause cracks in plastic working on the cast product. In order to eliminate such surface defects of the cast product generated in casting, it has been necessary to perform surface treatment on the cast product, such as removal of cracks, surface cutting, surface dissolving, or the like, before the cast product is subjected to plastic working.
Further, in the case of casting an alloy such as cast iron or phosphorus bronze which has a wide range of solidification temperature, it has been impossible to draw out the cast product without generation of surface cracks, unless the cast product is intermittently drawn out from the mold after the molten metal has been completely solidified in the mold.
The conventional method for horizontal continuous casting has been a method in which a solidified shell is formed on the inner wall surface of such a cold mold, so that crystals constituting the solidified shell have been apt to grow columnlike in the direction substantially perpendicular to the wall surface of the mold. If a columnar crystal zone is formed in the surface layer of a cast product, cracks may be easily caused, from the crystal grain boundary, by friction between the cast product and the inner wall surface of the mold when the cast product is drawn from the mold. Further, in a cast product in which such a columnar crystal zone is formed in the outer circumference thereof, surface cracks may be easily caused in plastic working. Particularly, in the case of casting a metal or an alloy having poor workability, it has been considered that, even if such a metal or an alloy is cast into a cast product through continuous casting, it is difficult to make the cast product into a plate or a wire through plastic working.
In Japanese Patent Post-Examination Publication No. sho-55-46265 published Nov. 21, 1980, the inventor of this application proposed a novel continuous casting method with objects that such a surface solidified shell as described above is prevented from being formed on the inner wall surface of the mold, that crystals have a completely unidirectional solidified structure grown only in the direction of casting, and that surface defects due to friction between the cast product and the mold are prevented from being generated, thereby obtaining a metal molding having a smooth surface and having a desired cross section. The invention disclosed in the above Japanese Patent Past-Examination Publication No. Sho-55-46265 has been granted as Japanese Patent No. 1049146. This novel continuous casting method is a method in which a hollow mold is heated by a heating element so that the temperature of the inner wall surface at the outlet of the mold is to a value not lower than the solidification temperature of a casting metal, whereby a molten metal supplied from a moltenmetal holding furnace does not form any solidified shell on the inner wall surface of the mold, but a not-solidified molten metal on the surface of a cast product is started to be solidified at the outside of the outlet of the mold to thereby obtain a metal cast product having an unidirectional solidified structure elongated in the direction of casting through continuous casting.
In the case where the novel continuous casting method is applied to the horizontal continuous casting method described above, however, there is a possibility of occurrence of breakout of a molten metal at an outlet end of the mold depending on fine changes in temperature of the inside of the mold, in temperature of cooling water, and in rate of casting because solidification of a cast product is performed in the vicinity of the outlet of the mold. Accordingly, it is very important to always accurately grasp the position and shape of the solidified boundary surface in the mold.
In U.S. Pat. No. 4,605,056 granted on Aug. 12, 1986, therefore, the inventor of this application proposed a method for horizontal continuous casting of metal moldings in which the position of a solidified boundary surface can be accurately grasped by opening an upper portion of a hot mold, and an apparatus for realizing this method. According to this horizontal continuous casting method, a hot mold having a concave section opened in its upper surface is horizontally provided, in place of the hollow hot mold, on a side wall of a molten metal holding furnace just under the surface of the molten metal, a molten metal is caused to flow into the hot mold, and after the top end of a metal molding dummy previously set in the mold comes in contact with the molten metal, the dummy is drawn out of the mold and passed through a cooling means provided outside the mold so that the dummy and the metal molding following the dummy are cooled. If the mold is heated by a heating element provided on the mold so as to keep the temperature of the inner wall surface of the mold to a value not lower than the solidification temperature of the casting metal, the metal molding in the mold is not started to be solidified on the inner wall surface of the mold but is started to be solidified with priority only at the front end of the metal molding or on the rear end of the dummy with respect to the direction of movement of the metal molding, so that the metal molding can be drawn out continuously following the dummy as the dummy is drawn out to the outside of the mold. Accordingly, it is possible to continuously obtain a metal cast product having a smooth outer circumferential surface, having no nest, and having a unidirectional solidified structure elongated in the direction of casting.
However, it has been found that in order to cause the metal molding in the hot mold not to start to be solidified on the inner wall surface of the mold but to start to be solidified only at the front end of the metal molding or the rear end of the dummy with priority, it is necessary to drawn out the dummy from the hot mold at a fixed low rate, while if the drawing rate is made high, there is a possibility that the metal molding cannot adhere on the dummy or the not-yet solidified molten metal may flow as it is out of the outlet of the hot mold.
In U.S. Pat. No. 4,789,022 granted on Dec. 6, 1988, the inventor of this application further proposed a method in which a molten metal is supplied from a nozzle onto a solidification support heated to a value not lower than the solidification temperature of casting metal, and in which the solidification support is moved at a constant speed to thereby draw out a metal molding. The solidification support is formed like an endless belt so that the molten metal is supported on the belt surface, or like a rotary drum so that the top end of the nozzle is caused to come close to an outer circumferential surface of the rotary-drum-like solidification support so as to supply the molten metal onto the outer circumferential surface of the support, whereby the molten metal is prevented from flowing in the direction opposite to the direction of movement of the solidification support.
In such a configuration, the solidification support is heated to a temperature not lower than the solidification temperature of the metal, in the vicinity of the position at which the molten metal is supplied to the solidification support from the nozzle, while the solidification support is also cooled at a cooling portion. Accordingly, the solidification support is heated, cooled, and then heated. That is, the solidification support is subject to repetition of heating and cooling so that it has a defect that it is apt to deteriorate. Specifically, in the case of casting a metal, such as aluminum, copper, ion, or the like, having a high melting point, it has been found that the difference between the heating temperature and the cooling temperature is so large that cracks may occur in the solidification support in a short time or the surface of the solidification support comes off from the body thereof to thereby make the solidification support unusable. Accordingly, the method has been used only for casting a metal, such as tin, zinc, or the like, having a low melting point. Further, the nozzle is arranged so that its top end is close to the solidification support in order to make the molten metal not to flow in the direction opposite to the direction of movement of the solidification support. Since the gap between the top end of the nozzle and the solidification support is so small that the top end of the nozzle sometimes comes into contact with the solidification support with the movement of the solidification support. Accordingly, there is a possibility that the contact causes friction or the like to thereby widen the gap between the top end of the nozzle and the surface of the solidification support to cause so-called breakout so that the molten metal flows out from the gap in the direction opposite to the direction of movement of the solidification support. Further, in the case where the solidification support is made of a material, such as graphite, which may be consumed through oxidization, it is necessary to shield the whole of the solidification support from the air, and a shielding device such as a cover or the like can not but being made large-sized. Accordingly, not only the apparatus becomes expensive but the maintenance and inspection thereof become troublesome.